1. Field of the Invention
The present invention is related to an image compression device by which color still image data is compressed in accordance with an Hadamard's transformation, and also to an image expansion device by which the compressed image data is expanded in accordance with an inverse Hadamard's transformation.
2. Description of the Related Art
A standard algorithm, for the encoding of high resolution images and the transfer of that information through telecommunication transmission channels, has been recommended by the JPEG (Joint Photographic Expert Group). In the JPEG system, a large-scale data compression is performed as follows: the original image data represented by pixel values are divided into a plurality of 8.times.8 blocks, each of which is subjected to a two dimensional discrete cosine transformation (DCT), so that the original image data are broken down into components on a spatial frequency axis. The components expressed on the spatial frequency axis are quantized by using a quantization table, and are then encoded to become compressed image data.
Although the image data are compressed with a high efficiency by the JPEG system, the image compression is performed using a non-reversible method in which an error exists between the original image data and reproduced image data, which are obtained by expanding the compressed image data. Conversely, for a reversible method in which no error exists between the original image data and the reproduced image data, an image compression using an Hadamard's transformation can be utilized. In the Hadamard's transformation system, the image data is converted or inversely-converted using a symmetric matrix, which is constructed only of components 1 and -1, and the Hadamard's transformation can be performed only by addition and subtraction calculations.
In the Hadamard's transformation, however, in comparison with the JPEG system, the compression ratio is low, and thus the amount of compressed image data is large, which presents a disadvantage when recording and transmitting the compressed image data.